Due to the rapid deployment of the Synchronous Optical Network (SONET) and the similar Synchronous Digital Hierarchy (SDH) standards for broadband transmissions, switching systems of the future are likely to have greater capacity and a wider variety of service types integrated together. These new services are envisioned to include more extensive video conferencing than is presently deployed, due to advances in video compression based on the P.times.64 standard (where P is an integer). This standard is based on the Discrete Cosine Transform (DCT), which allows varying levels of quality based on multiples of 64 kb/s. Business video conferencing will place a burden on new switching systems to have broadcast capability. However, the same DCT algorithms are being developed for still-image transfer (known as the JPEG standard) and full-motion broadcast video (known as the MPEG standard). Several manufacturers are developing chip sets based on these standards, which are envisioned to eventually be deployed in most television sets, personal computers, and workstations. The resulting explosion in the consumer markets along with the business markets will place heavy demands on switching systems of the future to be able to handle not only variable bandwidth (bandwidth on demand), but also extensive broadcast trees.
Because transmissions will be based on multiples of 64 kb/s as part of the hierarchical nature of the SONET and SDS formats, time-division switching fabrics will lend themselves well to this task. However, in multi-stage fabrics, the extent to which this format can be exploited depends on the ability of the switch-control mechanism to be able to hunt paths rapidly. For example if the fabric is carrying broadcast video, call-hold times may be very short due to customers browsing through the available channels. Another scenario requires rapid setup and tear down of broadcast trees in a video conference in order to follow changes in whoever is the active speaker. The demands made on software-based path hunting may be overwhelming.
Another consideration implicates the non-blocking performance of the switching fabric. A switching-fabric architecture that is guaranteed to be non-blocking in the strict sense for point-to-point connections is not necessarily non-blocking when the mix of switched traffic includes broadcast (including multicast) connections, and the probability of blocking generally increases with the number and the fanout of the broadcast connections.
One promising algorithm involves branching each broadcast call as close to the output of the switching fabric as possible. This results in the use of fewer resources at the input side of the fabric, thus making them available for new calls. Interestingly, the conservation of resources resulting from this broadcasting algorithm actually decreases the probability that a new call will be blocked; as a consequence, the network performance is better, on average, when broadcasting is occurring. Unfortunately, the complexity of implementing this algorithm by means of conventional path-hunting techniques is likely to result in a cumbersome path-hunt arrangement that is too slow to meet the rapid path setup and tear down requirements that were discussed above.